In conventional radio frequency (RF) plasma processing, such as that used during stages of fabrication of many semiconductor devices, RF energy may be provided to a substrate process chamber via an RF energy source. The RF energy may be generated and provided in continuous or pulsed wave modes. Due to mismatches between the impedance of the RF energy source and the plasma formed in the process chamber, RF energy is reflected back to the RF energy source, resulting in inefficient use of the RF energy and wasting energy, potential damage to the process chamber or RF energy source, and potential inconsistency/non-repeatability issues with respect to substrate processing. As such, the RF energy is often coupled to the plasma in the process chamber through a fixed or tunable matching network that operates to minimize the reflected RF energy by more closely matching the impedance of the plasma to the impedance of the RF energy source. The matching network ensures that the output of the RF source is efficiently coupled to the plasma to maximize the amount of energy coupled to the plasma (e.g., referred to as tuning the RF power delivery). Thus, the matching network ensures that the total impedance (i.e., plasma impedance+chamber impedance+matching network impedance) is the same as the output impedance of the RF power delivery. In some embodiments, the RF energy source may also be capable of frequency tuning, or adjusting the frequency of the RF energy provided by the RF energy source, in order to assist in impedance matching.
In process chambers that use multiple separate RF power signals pulsed at multiple power levels, synchronized RF pulsing is typically used. The multiple separate RF power signals may be pulsed independently out-of-phase with each other, or with varying duty cycle. Synchronization may be accomplished through the use of transistor-transistor logic (TTL) signals. One master generator creates the TTL signal to the other slave generators for synchronization. Each RF generator (masters and slaves) can provide pulsed RF power at independent duty cycles and/or pulse delays.
However, in RF single level pulsing (SLP) or dual level pulsing (DLP) using multiple separate RF power signals pulsed at multiple power levels (e.g., each with high/low power settings), the multiple impedance changes that occur during a pulse duty cycle makes impedance tuning difficult. That is, the match network and/or RF generators cannot adequately tune for the reflected power as the reflected power changes multiple times within each duty cycle. More specifically, RF matching can typically only tune to one impedance and cannot tune both impedances to 50 ohm at the same time.
Accordingly, the inventors have provided improved methods and apparatus for RF pulse reflection reduction in process chambers that use multiple separate RF power signals, pulsed at multiple power levels.